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Full text of "Text Book Of Mechanical Engineering"

872                            Appendix II.

I    "'

;*                              the screw taken at f- the I.H.P.    The direct thrust may be found

from speed, for

V of ship = knots x 101*3

Thrust x knots x 101-3 = effective H.P. x 33000
I                               Qr p = (eff.H.P.)x 33000 ^LHjP,

K x Tor3                     K

- „         f            .    ,     217 I.H.P            I.H.P.

.-. Collar surface required =-------~— = 4*34 ———

50 JtL                         jL

It must be noted that these surfaces' are now horseshoe in
form (see p. 691) and more collars are required than if circular.

For pivot bearings the experimental apparatus in Fig. 833
was adopted, where B is the pivot or footstep fed by oil entering
at pipe H. The shaft D being rotated through bevel gearing A A,
the frictional moment was measured by weight G, which, acting
on pulley F, prevented the bearing at B from rotating. At the
same time, the load was obtained by oil pumped against surfaces
D and E, its intensity being measured by pressure gauge c.

P. 568. Balls arid Live Rollers.—It is found that with
ball or roller bearings the frictional loss is £ or $ of that of a
plain journal, and in large bearings the rollers are kept apart by.
rings, as in Fig. 5 84.

P.575. Efficiencies of 'Machines.—The example on
pp. 571 to 575 shews the methods usually followed in mecha-
nical laboratories to find frictional loss in machines. Two
further cases may be given by way of illustration. Fig. 834 is
a chart of experiments on rope-pulley blocks, a simple fixed
pulley being called a ; i : i system, one movable and one fixed
block a 2:1 system, and so on. Thus 3 upper and 3 lower
pulleys make a 6: i system. Plotting P : W the inclined lines
are obtained, and efficiency curves are further calculated for each
load, A curve of maximum efficiency is also drawn belgw, on a
base of velocity-ratio, which usefully indicates the rapid loss with
increased theoretical advantage. The sheaves were 2§" diameter
and the rope J".
•*                Investigating, we have in a i: i system :

P = W(r +m)       where m is a proper fraction.